As shown in FIG. 1, a conventional isolated power converter 10 includes a rectifier circuit 12 to convert an alternating-current (AC) voltage VAC to a direct-current (DC) voltage Vin applied to a primary coil Lp of a transformer T1 via a snubber 16, a power switch 18 serially connected to the primary coil Lp, and a controller 14 to generate a control signal Vgate according to a feedback signal Vcomp and a sense signal Vcs proportional to the current Ip flowing through the primary coil Lp, to switch the power switch 18 to convert the voltage Vin to an output voltage Vout. The controller 14 has a power input pin VDD to receive a supply voltage Vcc. A feedback circuit 20 detects the output voltage Vout to generate the feedback signal Vcomp for the controller 14. The feedback circuit 20 includes an opto-coupler 22 and a Zener diode 28 functioning as a shunt regulator. The opto-coupler 22 generates a current Icomp according to the output voltage Vout to determine the feedback signal Vcomp. The opto-coupler 22 includes a light-emitting diode (LED) 24 as an input device and a transistor 26 as an output device. A current Id proportional to the output voltage Vout flows to the ground via the LED 24 and the Zener diode 28. The opto-coupler 22 amplifies the current Id flowing through the LED 24 to generate the current Icomp flowing through the transistor 26. The Zener diode 28 is connected to the LED 24 to limit the maximum voltage at the cathode of the LED 24.
When the loading of the power converter 10 becomes lighter, the output voltage Vout increases and thereby causes an increase in the current Id flowing through the LED 24. As a result, the current Icomp flowing through the transistor 26 increases accordingly. The increased current Icomp pulls the feedback signal Vcomp lower and thereby shortens the on time of the power switch 18. However, the increase in the currents Id and Icomp also implies higher power consumption, which lowers the efficiency of the power converter 10 at light loading.
Therefore, it is desired an apparatus and method for improving the light load efficiency of an isolated power converter.